1. Field of the Invention
This invention relates to a switching power circuit, more particularly to a switching power circuit with a redundant structure used in a telecommunication device.
2. Description of the Related Art
In a conventional switching power circuit with a redundant structure, diodes such as a Schottky barrier diode (SBD) or the like have been generally used as a back-current obstructing element, however, since the diodes fundamentally cause a constant voltage drop, power loss is large and power conversion efficiency is low. Also, it is proposed that a field effect transistor (FET) be used for the back-current obstructing element as means for improving the above.
As one example, a switching power circuit disclosed in for example, Japanese Patent Unexamined Publication No. Hei 1-107656 will be described with reference to FIG. 1.
As shown in FIG. 1, the conventional switching power circuit includes a primary winding 22a of a voltage conversion transformer 22 and a switching transistor 23 both of which are connected in series with an input power source 21a, rectifying diodes 24 and 25 connected to a secondary winding 22b of the voltage conversion transformer 22, a smoothing filter consisting of a choke coil 29 and a capacitor 30, an FET 31 connected to a usual forward converter consisting of a voltage control circuit 32 and a load 33, and a rectifying circuit consisting of a capacitor 27 and a bias resistor 28.
The FET 31 is interposed between the load 33 and the smoothing filter consisting of the choke coil 29 and the capacitor 30 in such a manner that a source electrode of the FET 31 is connected to an output terminal of the smoothing filter, a drain electrode thereof is connected to the load 33 and a gate electrode thereof is connected to a diode 26 connected to the secondary winding 22b of the voltage conversion transformer 22 and the rectifying circuit consisting of the capacitor 27 and the bias resistor 28. Reference numeral 34 denotes a switching power circuit with a redundant structure of another system, which is a switching power circuit with the same structure of that having structural elements of the input power source 21a to the voltage control circuit 32.
The operation of the conventional switching power circuit thus organized will be described.
First, in a state where the switching power circuit is operated normally, the output voltage of the diode 26 is set to be twice or three times as large as the output voltage of the switching power circuit according to the duty condition peculiar to the switching power circuit, and a voltage between the gate and the source of the FET 31 is equal to or twice as large as the output voltage of the switching power circuit so that the FET 31 is satisfactorily turned on. Then, the voltage drop of the FET 31 is expressed by the following expression (1), and the voltage drop can be remarkably reduced by selecting an FET having a low on-state resistance R.sub.ON as the FET 31, in comparison with the use of usual diodes, thereby being capable of reducing power loss. EQU V.sub.DS =R.sub.ON .times.I.sub.OUT ( 1)
where V.sub.DS is a voltage between the source and the drain and I.sub.OUT is an output current.
When the switching power circuit is not operated because of some trouble, switching off or the like, the induced voltage of the voltage conversion transformer 22 becomes zero (0), as a result of which the gate potential of the FET 31 is clamped to zero due to the bias resistor 28, thereby turning off the FET 31. The conventional switching power circuit has the disadvantages that the gate driving speed of the FET is low and the output current flows into the switching power circuit at the time when the rectifying diode or the like is short-circuited, thereby enlarging the fluctuation of the output voltage.